This disclosure relates to neutral layer polymers for self-assembled structures, methods of manufacture thereof and to articles comprising the same. In particular, the present disclosure relates to embedded neutral layers that are produced on a block copolymer and that facilitate the production of domains that are perpendicular to the substrate.
Block copolymers form self-assembled nanostructures in order to reduce the free energy of the system. Nanostructures are those having average largest widths or thicknesses of less than 100 nanometers. The self-assembly produces periodic structures as a result of the reduction in free energy. The periodic structures can be in the form of micro domains, for example, lamellae or cylinders. Because of these structures, thin films of block copolymers provide spatial chemical contrast at the nanometer-scale and, therefore, they have been used as an alternative low-cost nano-patterning material for generating periodic nanoscale structures. While these block copolymer films can provide contrast at the nanometer scale, it is however often very difficult to produce copolymer films that can display periodicity at less than 20 nanometers. Modern electronic devices however often utilize structures that have a periodicity of less than 20 nanometers and it is therefore desirable to produce copolymers that can easily display structures that have average largest widths or thicknesses of less than 20 nanometers, while at the same time displaying a periodicity of less than 20 nanometers.
Many attempts have been made to develop copolymers that have average largest widths or thicknesses of less than 20 nanometers, while at the same time displaying a periodicity of less than 20 nanometers. The following discussion details some of the attempts that have been made to accomplish this.
FIGS. 1(A) and 1(B) depict examples of lamella forming block copolymers that are disposed upon a substrate. The block copolymer comprises a block A and a block B that are reactively bonded to each other and that are immiscible with each other. The lamellae can align their microdomains to be either parallel (FIG. 1(A)) or perpendicular (FIG. 1(B)) to the surface of a substrate upon which they are disposed. The affinity of the block A and/or block B for the surface of the substrate determines the morphology on the substrate surface. Likewise, the affinity of the block A and/or block B for air determines the morphology at the air-block copolymer interface. The air-block copolymer interface is termed the free surface. The lamellae can also align their microdomains to be both parallel and perpendicular to the substrate (FIG. 1(C)). In the FIG. 1(C), the lamellae of block A are perpendicular to a plane parallel to the substrate surface, while being parallel to the substrate at the upper surface that contacts air.
The perpendicularly oriented lamellae provide nanoscale line patterns, while there is no nanoscale surface pattern created by parallel oriented lamellae. Where lamellae form parallel to the plane of the substrate, one lamellar phase forms a first layer at the surface of the substrate (in the x-y plane of the substrate), and another lamellar phase forms an overlying parallel layer on the first layer, so that no lateral patterns of microdomains and no lateral chemical contrast form when viewing the film along the perpendicular (z) axis. When lamellae form perpendicular to the surface, the perpendicularly oriented lamellae provide nanoscale line patterns. Therefore, to form a useful pattern, control of the orientation of the self-assembled microdomains in the block copolymer thin films is desirable.
With reference to the FIG. 1(C), in order to expose the perpendicular lamellae to the air interface, the uppermost layer (identified as being a layer of the B block) is etched to expose both A and B microdomains to the free surface. The presence of both microdomains of the A block and the B block at the free surface (with both being perpendicular to the substrate) provides nanoscale line patterns that can be used for nano-patterning (i.e., the development of templates and resists (e.g., photoresists) for the development of semiconductors). In short, when the free surface interactions are unbalanced, a skin layer forms of the block with the lowest surface energy.
External orienting factors are often used to facilitate orientation of the block copolymer microdomains. Without external orientation control, thin films of block copolymers tend to self-organize into randomly oriented nanostructures with undesired morphologies, which are of little use for nano-patterning because of the random nature of the features. Orientation of block copolymer microdomains can be obtained by guiding the self-assembly process with an external orientation biasing method. Examples of this biasing method include the use of a mechanical flow field, an electric field, a temperature gradient, by using a surface modification layer upon which the block copolymer is disposed, or by adding a surfactant to the copolymer. The copolymers generally used for these particular form of guided self-assembly are polystyrene-polymethylmethacrylate block copolymers or polystyrene-poly(2-vinylpyridine) block copolymers.
The FIG. 2 details one method of using a surface modification layer upon which a block copolymer is disposed to produce a film having controlled microdomain sizes, periodicity and orientation. The method depicted in the FIG. 2, has been previously detailed by P. Mansky, Y. Liu, E. Huang, T. P. Russell, C. Hawker, Science 275 (1997), 1458. As with the FIG. 1, the block copolymer of the FIG. 2 comprises a block A and a block B. The substrate in the FIG. 2 is coated with a surface modification layer that is affixed to the surface. The surface modification layer is formed by crosslinking or is reactively bonded (covalently, ionically or hydrogen bonded) to the surface of the substrate. Any additional excess material is removed prior to or during the bonding. The block copolymer is then coated on the surface modification layer of the substrate.
Surfactants can also be an external orienting factor that can be used to control free surface interactions. Oleic acid has been used as a surfactant when casting a block copolymer comprising polystyrene and polymethylmethacrylate. The perpendicular morphology has been found to persist over a range of thicknesses when the copolymer was cast on non-neutral substrates. The block copolymer after being cast on a substrate is optionally annealed with heat (in the presence of an optional solvent), which allows for phase separation of the immiscible polymer blocks A and B. The annealed film can then be further developed by a suitable method such as immersion in a solvent/developer or by reactive ion etching which preferentially removes one polymer block and not the other to reveal a pattern that is commensurate with the positioning of one of the blocks in the copolymer.
While these particular methods of external orienting produce block copolymers, some of them are expensive (e.g., reactive ion etching) while others (e.g., the use of a surfactant) leave behind residues that make their use impractical.